Bug fix & documentation: KernelPCA w/ precomputed X

[eahenle]

Bug: fit(KernelPCA, X; kernel=nothing) mutates X via K = X prior to calling transform! on K Fix: K = copy(X)

Documentation also does not mention the use of kernel=nothing when X is a pre-computed Gramian; added this.

[eahenle]

Also may be worth documenting the fact that fit applies kernel centering (so the user needn't center precomputed kernels). Kind of harmless to leave this undocumented, but also easy enough to mention in the docs.

[wildart]

You fixed the old doc file. Please, fix doc string for fit function.

[eahenle]

I have added the explanation of kernel=nothing to the docstring

[wildart]

Thanks.